Beam tube storage system



April l2, 1955 H. KLEMPERER BEAM TUBE STORAGE SYSTEM 2 Sheets-Sheetl 1 Filed Feb. l1, 1948 IIIII im RE M Haifl SGG .Sim

April 12, 1955 H. KLEMPr-:RER

BEAM TUBE STORAGE SYSTEM 2 Sheets-Sheet 2 Filed Feb. l1, 1948 mummtnm, NQ@ mwmmm. to wumwk bwwmkmww Q /v m.

SEE. @REQ @SQ SSS QQQT @momu QEESWEQQ- @E REQQ R@ QkMUN M LONQM\|\MU I I l I l l I I l l Il @ffm/2510@ /NVENTOR HANS KL Empa/PER United States 2,706,246 Fatented Apr. 12, 1955 tice BEAM 'rusa sronaon SYSTEM Haus Klemperer, Beixnont, Mass., assigner to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application February il, 1948, Serial No. 7,645

Claims. (Cl. Z50-27) This invention relates to electrical circuits, and more particularly to a circuit by means of which a periodicallyrecurring weak signal can be detected when mixed with, and can be segregated from, a non-periodic background wave which is of larger amplitude.

In substantially all electrical circuits using electron discharge tubes, the factor which establishes a minimum signal amplitude level below which signals cannot be detected is the noise level inherent in the overall system; that is to say, if a signal has an amplitude or strength which is lower than that of the overall inherent system noise, such a signal cannot ordinarily be detected no matter to what extent it is amplified, since the noise is also amplified an equal amount `and will continue to overshadow or cover up even the amplied signal.

Heretofore, in order to detect weaker and weaker signals mingled with inherent noise, it has been necessary to increase the power expenditure of the signal transmitting station, in order to raise the level of the signal as received at the signal receiving or detecting station above the inherent overall noise level of the components of the latter station. rlfhis procedure is wasteful.

Therefore, an object of the present invention is to devise a circuit by means of which a weak input signal, one which is below the overall noise level of a signal receiving station, may be detected.

Another object is to accomplish the aforesaid object without the necessity of increasing the power expended, thereby enabling the same system sensitivity to be obtained with a reduction of power as compared to previous systems or circuits.

A further object is to provide means for detecting a periodically-recurring weak signal in the presence of a non-periodic background Wave of larger amplitude.

The foregoing and other objects of the invention will be best understood from the following description of an exemplilication thereof, reference being had to the accompanying drawings, wherein:

Pig. l is a diagrammatic representation of a system embodying the invention;

Fig. 2 is a charge-potential diagram useful in explaining the operation of the invention; and

Fig. 3 is a pair of patterns useful in explaining the operation of the invention.

Now referring to the drawings, and more particularly to Fig. 1 thereof, reference numeral l generally designates a storage tube of the type disclosed in my copending application, Ser. No. 787,873, filed November 25, 1947. Such a tube is, according to the invention, utilized in connection with the visual reproduction or detection of a periodically-recurring weak signal in the presence of an aperiodically-occurring or random stronger signal, such as noise. It is to be understood that this invention is applicable to the detection of periodicallyrecurring sub-noise electrical signals of many different kinds and from many different sources. For purposes of illustration, out not for limiting this invention thereto, the invention will be disclosed in connection with the detection of a sub-noise signal due to the presence of a target in a given area scanned by a system employing electromagnetic signal reflection methods, or in connection with a radar system.

As used herein, the term ,sub-noise signal refers to avsignal having an amplitude which is below or less than that of the noise level inherent in the electronic portions of the detecting and amplifying system.

Mounted in the narrow end of the envelope of tube l is an electron gun comprising a control grid 2, first, second and third anodes 3, 4 and 5, respectively, and a cathode 6 for producing the necessary electron population when suitably energized. A source of potential 7 provides the necessary focusing and accelerating potentials to the anodes 3 5, in a manner familiar to those versed in the art pertaining to this type of electron discharge device. Thus, anodes 3 and 5 are grounded at 8, the positive terminal of source 7 being grounded at 9 and the negative end of said source being connected directly to cathode 6. Anode 4 is connected by means of tap 1t) to an intermediate point on source 7. Under these conditions of energization, provided that cathode 6 is heated by any suitable means (not shown) and provided that coutrol grid 2 is not biased negatively beyond beam cutoff, an electron stream 11 is projected from cathode 6 toward the right through anodes 3 5 along the axis of the envelope toward the enlarged right-hand end thereof.

A pair of vertical deecting plates 12 and 12A is mounted in the envelope to the right of anode S in -a position to act on the stream 11. Plate 12 is grounded as at 13, one output terminal of a synchronizable source 14 of time sweep voltage being also grounded as at 15. Plate 12A is connected to the other output terminal of source 14, so that stream Sil is deflected horizontally back and forth in synchronism with the output voltage of source 14 in a manner familiar to those skilled in the art of cathode ray oscillography. Suitable focusingcoils (not shown) are preferably provided in order to focus stream l1 into an electron beam of substantially rectangular cross-section. A conductive coating 16 inside the tube envelope is grounded by being connected to anode 5 and serves to electrically shield theY electron beam represented by 11.

Disposed at the enlarged end of the tube envelope opposite from cathode 6, in the path of projection of beam 11, is a potential storing target surface 17. This electrode 17 is a plate of electrically insulating or nonconduoting material, such as glass, and is backed by a conductive coating 18 which serves as the so-called repeller electrode and by means of which a negative bias voltage may be applied to surface 17 to bias said surface negatively with respect to ground. In order to so bias said surface, coating 18 is connected through a resistor i9 of high resistance to the negative terminal of a source of potential 2t), the positive terminal of which is grounded at 21. The potential of source Ztl is preferably on the order of 500 to 1000 volts. Electrode i7 is a substantially planar surface, the plane of said electrode lyingy at substantially right angles to the axis of beam 11, as shown.

An electrode 22 is substantially planar and is parallel to electrode 17, being spaced from said electrode and in front of the same. Electrode 22 is preferably constructed of nickel wire of very small diameter in the form of a 1GO-mesh screen and is disposed perpendicularly to electron beam 11, said electron beam passing through screen electrode 22 and impinging on the electrode 17.

Control grid 2 may be used to control the intensity of the electron beam il or to control the beam current of said beam, and in this invention the signals to be detected are applied to this grid to modulate the intensity of the said beam in accordance with the signals to be detected. In order to provide a low predetermined rest or no-signal intensity of the electron beam 11, a fixed negative potential bias is applied to grid 2 during the writing operations. The positive side of a source 23 of bias potential is connected to cathode 6, the negative side of said source being connected through a resistor 24 and a fixed contact 25 and the movable contact 26 of a gating circuit or controllable switch 27 (when said switch has the position illustrated) to grid 2; by this means grid 2 is negatively biased during the writing operations of the beam.

in order to modulate or vary the intensity of the elec tron beam 11 from its predetermined rest or no-signal intensity, and particularly to increase the intensity of said beam from said rest intensity, set by the voltage of source 23 during the writing operation, in response to the signals to be detected, a source 28 ot' positive periodic input signals, suchv as a radar receiver, has its output connected through a condenser 29 to fixed Contact 25, and thereby also to grid 2 when gating circuit 27 has the position il1ustrated. Radar receiver 28 is shown in block form because such a receiver is conventional and is familiar to those skilled in the radary art. rthe output of receiver 28 is a series of pulses corresponding to those reflected from reflecting objects in space within the iield of search of the radar equipment. Since these pulses are reected wave energy, for each and every particular target, they are periodic, the periodicity of such pulses being the same as that of the pulses transmitted toward such target.

Cathode 6, being connected to the negative end of source 7, the positive end of which is grounded, has a negative potential with respect to ground. The potential of source 'i is quite high, on the order of 160() volts, for example.

A source Si? of synchronizing voltage pulses serves as a source of radar transmitter pulses. the synchronizable source 14 of time sweep voltage is triggered by source 30, at the time of each transmitted pulse, to begin a new horizontal sweep of beam 11 across target 17 at the time of' each transmitted pulse and synchronously therewith. sweeps ot the electron beam is equal to the neriodicity of the signals from source 28, which signals are the ones to be detected.

Collector screen 22 is connected through a resistor 32 to ground at 33, so that said screen is at ground potential. Opposite ends of load or output resistor 32 are connected to provide the input to a gated output amplifier 34, a direct current blocking condenser 3:3 being provided in series in one of the leads to amplifier 3a. The amplified output of amplifier 3ft maybe applied to an oscilloscope i' 36, as shown. Amplifier 34 is normally gated olf but is capable of being gated on by a video gate pulse supplied thereto through a connection 37.

T he secondary emission ratio ot' an electron target,

whether an insulator or a conductor, depends upon the voltage of the incident electrons. The secondary emission ratio of such a target may be defined as the ratio of the secondary emission current leaving the target to the primary electron current striking the target, and for each material there is a certain value V1 of primary electron voltage which gives for such material a secondary emission ratio of unity. When the voltage of the primary electron beam is less than the value V1, the secondary emission ratio is less than unity and the number of electrons striking the target exceeds the number leaving the target, while, when the voltage of such beam is greater than V1,

By a connection 31, f

Therefore, the periodicity of.

the secondary emission ratio is greater than unity and the number of electrons leaving the target exceeds the number striking the target.

For explaining the action of the tube, we will disregard the negative potential of the repeller 18 for an instant. The electron beam is of high voltage with respect to ground or with respect to-plate 17 if said plate is at ground potential, this high voltage being on the order of 1600 volts, for example. This voltage is substantially greater than the so-called critical voltage Vr defined above, which critical voltage gives a secondary emission ratio of unity for target 17. Therefore, the number of secondary electrous leaving the target 17, from the areas bombarded by the electron beam 11, is in excess of the number of primary electrons striking said target in such areas. Since more secondary electrons are leaving the target than are striking it, a net positive charge or voltage tends to be produced, on plate 17 in the areas bombarded by the electron beam 11.

lf the intensity ot the beam is made sufficiently high, with a given sweep or writing speed, an equilibrium potential is produced in a single sweep throughout the areas of the storage plate bombarded by the main electron beam. Such equilibrium condition results from the following action. As the surface of plate 17 builds up a positive potential, due to the secondary emission ratio being greater than unity, as described above, the positive charge on this plate tends to attract the secondary electrous back to the plate, since such electrons have a rather slow velocity. This retarding or attracting effect on the secondary electrons increases as the voltage of plate 1'7 becomes more positive, until a point is reached at which the number of secondary electrons which succeed in escaping the retarding or attracting voltage of plate 17,

and which therefore leave said plate, equals the number of primary electrons striking the plate, giving a zero net target current at this equilibrium point. When this equilibrium potential has been reached throughout the trace or throughout the area of the storage plate bombarded by the electron beam, such trace or area may be termed saturated It has been found that this equilibrium condition or saturation is reached when the potential of plate 17 is on the order of two volts positive with respect to screen 22 or ground. An equilibrium condition or saturated trace will leave the storage surface 17 covered with an area of discrete charges at equilibrium potential, or having a predetermined value of potential with respect to ground, on the order of two volts positive, for example. For a somewhat more detailed explanation of the above action, reference may be had to my aforesaid copending application.

The above explanation has assumed an electron beam intensity sufficiently` high to produce this equilibrium potential or saturation or the trace in a single sweep at a given writing speed. lf the intensity of the electron beam is not this high, the trace does not reach the saturation value or the equilibrium potential in a single sweep at such writing speed, because in this case there are insufficient electrons in the beam to malte the voltage of the storage plate go sufficiently positive for equilibrium in a single sweep at a given speed of travel thereacross. Under these conditions, more than one sweep is necessary to produce a saturated trace. Variations in electron beam intensity cause corresponding variations in potential level of the target surface, because of the corresponding variations in electron density. According to this invention, the beam intensity is reduced to a value such that the beam writes a trace on the storage plate far below the saturation value in a single sweep, as will more clearly appear hereinafter.

During operation of the storage tube, the repeller 18, and thereby the whole storage plate 17, is raised to a high negative potential by means of potential source Zt?. Although in Fig. l several sources of potential are shown in the form of batteries, the usual practice is to provide these potentials from a source of suitably-rectied alternating current.

The potential of the storage plate 17 and repeller 1S relative to the potential of cathode 6 will determine the speed ofthe electrons striking the storage surface 17. ln the example given, the voltage of the repeller 18 may be i000 volts negative with respect to ground, while the voltage of cathode 6 may be 1600 volts negative with respect to ground, so that a difference of potential of 600 volts exists between cathode 6 and plate 17, cathode 6 being 600 volts negative with respect to plate 17. This voltage diiference is substantially greater than the so-called critical voltage V1 deiined. above, which critical voltage gives a` secondary emission ratio of unity for target 17; therefore, more secondaries are leaving than are arriving at the point of incidence of the electron beam onV the storage surface. Since this is so, the trace when saturated will still be slightly positive with respect to the potential of the collector 22, due to the above-explained action, so that for saturation the charge of the trace changes from the original unbombarded 1000 volts negative with respect to ground to two volts positive with respect to ground. For saturation, the beam trace produces an area on surface i7 throughout which the potential is uniform and predetermined, said potential being greatly different from the potential of those areas not bombarded by the electron beam because the areas not so bombarded remain at the negative 1000 volts potential (with respect to ground) applied to storage surface 17 by means of repeller 1S and potential source 20.

By biasing the storage surface 17 toa potential level which is substantially different from the predetermined equilibrium potential value or saturation potential value, l am able to pick up or detect periodically-re rring weak or sub-noise signals. rThis may be seen more clearly from a consideration of Fig. 2. This figure is a plot of various potential levels in. the storage tube, the zero potential level being assumed to be ground or the potentialv of electrode 22, as indicated by the ground symbol and the horizontal line labeled collector electrode. The cathode 6 has a potential of 1690 voltc negative, as indicated by the horizontal line labeied cathode The storage surface 17 and repeller 18 have a potential of 1000 volts negative, as indicated by the horizontal line escasas labeled repeller and storage surface. lt should be apparent, from a consideration of the above discussion, that the beam-bombarded portion of the target or storage surface tends to charge positively with respect to the -lOOO Volts bias potential level toward the saturation potential level, which is two volts positive, since the net voltage of the electron beam is greater than the critical voltage V1 previously deiined. For purposes of clarity, the two volts difference between ground and the saturation potential level is exaggerated in Fig. 2.

It may be seen, in Fig. 2, that there is a substantial difference of potential between the saturation potential level and the storage surface bias potential level, this potential diierence being on the order of 1000 volts. During recording or writing on the storage plate, the connections of the gating circuit are as illustrated in Fig. l. According to this invention, due to the application of a negative potential by means of source 23 to control grid 2 during writing, the intensity of the electron beam is reduced to a value far below that necessary or required to bring the beam-bombarded portions of target 17 to the predetermined equilibrium potential value or the saturation potential level during a single sweep of said beam thereacross. Therefore, in operation, the potential conditions along the bombarded portion of the target at the end of a single sweep may be as indicated in Fig. 2, with the signal potential level for one sweep and the noise potential level for one sweep both being somewhat positive with respect to the 1G00 volts bias potential level but still substantially below the saturation potential level. lt is desired to be pointed out that the signal potential level for one sweep is below the noise potential level for one sweep; in other words, the signal to be detected is below the noise level or is a sub-noise signal. Because of this very low writing electron beam intensity, a plurality of sweeps of the electron beam across the storage surface are required to cause the potential of the bombarded portion of the target, under any circumstances, to approach the saturation potential level.

It will be recalled that the signals to be detected occur periodically, with the periodicity of the signals being equal to the periodicity of the repetitive sweeps. Therefore, the signal repeats or occurs at exactly the same point on each sweep of the electron beam across the storage surface. Noise, on the other hand, is a random or non-periodic disturbance, so that such disturbance does not repeat at exactly the sarne point on each such sweep. Therefore, when successive sweeps are added to each other, it becomes possible to sum up the periodic signal at a faster rate than the random noise.

By the storage tube of this invention, successive sv yeeps or traces are stored and summed up. By applying the positive signal to be detected, and also the inherent noise, to the control grid 2 of the storage tube to increase the intensity of the electron beam il as compared to the rest or 11o-signal intensity of said beam, the successive sweeps produce areas of varying potential, the potentials varying in accordance with the instantaneous electron beam intensity responsive to the signals applied to grid 2. This intensity modulation of the electron beam produces a pattern of varying potentials in the bombarded area of the target surface.

Since the radar echo signal is periodic and repeats at exactly the same point in each sweep, and since the noise is random or aperiodic, the sub-noise signal is summed up at a faster rate than the noise, and the potential conditions along the bombarded portion of the target after a plurality of sweeps, for example, ten, may be as indicated in Fig. 2. At the end of ten sweeps, the signal potential level may have the value indicated by the corresponding legend, while the noise potential level D may have the value indicated by the corresponding legend. Since the signal sums up at a faster rate than does the noise, and since signal-produced increases in electron beam intensity cause corresponding increases in potential level of the corresponding points on the target surface over and above the no-signal positive charging of the beam-bombarded portion of the target with respect to its bias potential, the signal potential level is now greater than or above the corresponding noise potential level D as shown. Now, the signal potential level and the noise potential level D are both substantially positive with respect to the -1000 volts bias potential level but still somewhat below the saturation potential level, with the signal potential level substantially above the noise potential level. The summing-up effect of the potential level of the signal-representing point is indicated for each of the ten sweeps.

ln Fig. 2, only ten sweeps are represented as being utilized for the signal potential level to approach the saturation potential level. This has been done only in order to simplify the drawings; in practice, a far greater number of sweeps are preferably used. The greater the number of sweeps stored and summed up, the better is the sub-noise visibility. The number of sweeps necessary to reach saturation potential depends on both beam current and writing speed. As an example, with the repeller run at 1G00 volts and an electron beam current of JAC microampere, the gain in charge of each individual trace on the storage surface, depending on speed and spot concentration, will be of the order of ten volts. Neglecting leakage, which is justified if the succession of sweeps is rapid, about traces or sweeps would be needed to increase the trace charge to the 100G-Volt limit given by the repeller voltage.

The lower portion of Fig. 3 reports the integrated trace on the storage tube surface after the ten sweeps of Fig. 2. ln Fig. 3, the different densities of stippling represent the different corresponding densities of positive charges on the storage surface, or the different corresponding positive potential levels thereon. The random noise forms a charged background A which corresponds to the noise potential level D in Fig. 2. A pair of more-highly-positively-charged substantially rectangular areas B and C represent two sub-noise signals which have been brought up above the noise level by the summation operation of this invention.

A means is provided for reading ot the completed integrated or summed-up trace of Fig. 3 in a single sweep.

The gating circuit 27 is illustrated, for purposes of simplicity, as being a controllable switch having a movable contact 26 cooperatin0 selectively with one of a pair of xed contacts 25 and .59. The movable contact 26 is indicated as being controlled by a step and gate generator 40. Step and gate generator di) is controlled, in turn, from the impulse source 39 and functions to produce an output pulse in response to a predetermined number of input pulses from source 3i). This output pulse of generator 4l? has a duration which is equal to the time duration of a single sweep cycle of source 14, and occurs during what may be called a reading sweep of electron beam 11.

By connection 37, the output pulse of generator 40 gates on the amplitier 34 during this reading sweep; it will be recalled that this amplifier is oit except when so gated on.

Gating circuit 27 is preferably electronic, and said circuit is also gated by generator all. During the writing or recording process, during which the trace on storage plate 17 is being integrated or summed up as above described, circuit 2'7 is not gated, so that contact 26 is on contact 25 to connect signal source 28 to grid 2 and to bias said grid negatively by source 23| to reduce the beam intensity so that the beam writes a trace far below the saturation value, as previously described. The normal engagement of contact 25 with contact 25 is indicated by a spring 41 which biases contact 26 toward Contact 25.

When generator 40 produces an output pulse, circuit 27 is gated during said pulse to connect contact 26 to contact 39 during the time of this pulse (or during the time of the reading sweep) and after the end of this pulse contact 26 again is connected to contact 25. A potential source 38 has its negative side connected to cathode 6 and its positive side to contact 39, so that, when contact 26 is on contact 39, a positive potential is applied to control grid 2 of storage tube l. This positive potential on grid 2 produces a rather high-intensity electron beam il, the intensity being sufficiently high to bring the entire trace up to the predetermined equilibrium potential or saturation potential in the single reading sweep during which said positive potential is applied to grid 2.

It is intended to so operate the system of this invention that, after a predetermined number of sweeps, as shown in Fig. 2, the signal potential level will be substantially above the noise potential level, substantially above the 1000 volts bias potential of the storage plate, will be near to the saturation potential level, but yet will amends be somewhat below the saturation potential level. For any chosen predetermined number of sweeps, these conditions may be met by properly choosing the` electron beam current or beam intensity.

The predetermined number of sweeps, in response to whichy gate generator 40 produces an output pulse, is made equal to the said chosen predetermined number of sweeps d ened in the preceding paragraph. For Fig. 2, this wouldl be ten, and for the other example given above it wouldbe 100. After thisA number of sweeps, during which the low-intensity beam is integrating or summing up the same number of traces, in order to bring subnoise periodic signals up above the noise level, in the manner described above, gate generator e@ produces a pulse during the next or reading sweep to operate gating circuit 2 7 andrto gate onamplier 34 during said reading sweep. Ampliiier? 34 is gated off during the time of recordingy or summing upthe traces. The disconnection of contact 25 from Contact 25 disconnects the signal source 28 and the negative bias source 23 from the control grid 2 during this sweep, and the` connection of contact 26 to contact 39 connects the4 positive bias source 38 to said control grid during` this sweep.

During this reading sweep, therefore, the entire beambombarded portion of the target is brought up to the saturation potential level. During said sweep, different voltage changes will be produced on different elemental areasof the trace, the voltage change produced for any elemental area being the difference between the potential of such area at the end of the Writing sweeps and the saturation potential, which is twovolts positive. The higher the voltage level is for any particular area before the reading sweep, the smaller will be the output signal voltage for that area. Therefore, the sense of the output signal will be reversed as compared to. the input signal. However, this reversal may be compensated'for by having the proper number of amplifying stages in amplifier 34.

Collector electrode 22 is capacitively coupled to storage plate 17. Dielectric displacement, caused by the change of potential of the trace on plate 17 to the saturation potential level during the reading sweep in the manner explained previously, is transferred through interelectrode capacitances to collector 22. This produces a positive net collector signal or output voltage which is varying and which is amplied by amplifier 34, since said amplier is gated on during the reading sweep. Reference may be had to my aforesaid copending application for a more detailed explanation of thev output operatlon.

Now referring to Fig. 3, the curve E represents the output' to the indicator 36 during the` reading sweep. When the reading beam encounters areas A of low positive potential in the integrated trace, a minimum voltage is produced inthe output (assuming a 180 phase reversal in amplifier 34)', since here ay maximum voltage change is produced by the process of bringing such areasv up tol the saturation potential level by means ofthe read.- ing electron beam. When the beam encounters areas B and C of substantially higher positive potential in -the integrated, trace, which areas result from sub-noise periodic signals, corresponding `high-ampiitude,voltage pulses F and` G` are produced in the output, since here minimum voltage changes are produced by the process of bringing suchV areas up to the saturation potential level by means -of the reading electron beam.

After. the reading sweep, the saturation potential level of, the trace may be wiped offthe storage surface 17 in the usual manner by anysuitable means (not shown). A suitable meansrfor so wiping oif the trace is disclosedl in my copending application, Ser. No. 1,638, tiled January 10, 1948. This means for sweeping off the trace comprises4 aflood gun structure designated generally by the. numeral 'tl, Structure Si). consists of an elect-ronemissive cathode 51 which is capable of projecting; a broadbeam 52j of electrons through a control grid 53 anda foraminous anode 52% toward and to storage plate l?" on which an electron beam lll impinges. Cathode l isl connected to the negative end of a suitable source 51S of direct voltage, for example. a. battery, the positiveend of-'which is connected tof ground, so. that the llood gun;

cathode has a negativepotential withrespeetto ground.

The voltage ofthe lbattery. SSLisV-n which isless than:` the,

voltage Vi, the unitsecondary. emission ratio voitage for the target 17, which givesY for the material of. the

electron target 29 a secondary emission ratio of less than unity. The flood gun anode 54 is connected to ground at 56,.so that when the ilood gun is turned on, the entire storage surface 17 will be flooded with electron beam 52 whose voltage is less than the voltage V1 defined above. The flood gun controlgrid 53 is connected through a resistor 57 to the negative terminal of a biasing battery 58, the positive terminal of which is connected to the cathode S1. The voltage of battery 58 is such that grid 53 is normally biased to cutoff, so that beam 52 is normally cut off, but may be flashed on by applying an impulse of the proper amplitude and polarity to grid 53 by means of a lead 59 connected to said grid through a D. C. blocking condenser 62. Under the above-described voltage conditions, when the electron beam 52 strikes target 17, a negative charge is accumulated over the entire surface of the target, and a condition of equilibrium is nally reached at which the entire surface 17 is brought to acornmon potential. The beam current required to erase the storage surface charge, or

to bring the entire surface to a common potential, de.

pends on the length of the ood. gun beam pulse, the voltage Vf, and the capacitance between the storage surface 17 and the screen Z2.

A plurality of sweeps rnust be stored and summed up in order to selectively bring the sub-noise periodic signals up above the noise level. Therefore, the useful increase of sensitivity of the radar setV below the noise level is accomplished at the expense of speed of information, since the sweep impulses are transmitted to the indicator only at a rate of one impulse to the indicator for every ten or every impulses'transmitted from the radar transmitter.

The above completes the description of this invention. It is to be noted that the selective amplification of subnoise level signals by a storage tube according to this invention is made possible by two concepts, acting singly or in combination. One concept is the use'of a repeller electrode, by means ofl which a biasy may be placed on the storage surface, said bias being-substantially different from the saturation potential level, whereby a large number of sweeps of the electron beam across the storage surface mayl be effectuated before the trace approaches the saturation potential level. The other concept is the use of a very-low-intensity electron beam for writing,

said intensity being so low that with a given writing speed a trace far below the saturation potential level is written on the storage surface with a single sweep thereacross, whereby a plurality of sweeps are vrequired to bring the trace up to saturation or to a point near saturation.

Gf course, it is to be understood that this'invention is not limited to the particular detailsasv described above, as manyy equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad' interpretation commensurate with the scopeof. this invention within the art.

What is claimed is:

l. An electron discharge tube circuitr comprising an electron gun for projecting a beamof' electrons, a potential storing target surface in. the path of projectionl of said beam, means for applying a negative potential to said surface to initially bias the same, means lrepetitively sweeping said beam, across said surface at a predetermined speed, means for producing an electron beam volt- H age with respect to the bias potential of said surface in the region necessary to give a secondary emission ratio greater than unityfor said target so thatl the beam bombarded portion of said target tends to charge positively with respect to said bias potential: toward' arpredetermined equilibrium'potential value substantially diderent from said bias potential, means for controlling the Writing intensity of said beam to a value substantially below that required to bring the potential of said bombarded portion to said equilibrium value in a single sweep thereacross, whereby a predetermined number more than one of such sweeps are required to cause the potential of said bombarded portion to approach said equilibrium value, means effective after said predetermined number'of sweeps for sweeping `a high intensity reading electron beam across said surface to bring the potential of all of said bombarded portion to said equiV libriuzn value duringsaid last-named single, sweep, and means responsive to thesecondary emission of electrons produced by the high intensity reading electron beam for producing an output signal.

2. An electron discharge tube circuit comprising an electron gun for projecting a beam of electrons, a potential storing target surface in the path of projection of said beam, means for applying a negative potential to said surface to initially bias the same, means repetitively sweeping said beam across said surface at a predetermined speed, means for producing an electron beam voltage with respect to the bias potential of said surface in the region necessary to give a secondary emission ratio greater than unity for said target so that the beam bombarded portion of said target tends to charge positively with respect to said bias potential toward a predetermined equilibrium potential value substantially different from said bias potential, means for controlling the writing intensity of said beam to a value substantially below that required to bring the potential of said bombarded portion to said equilibrium value in a single sweep thereacross, whereby a predetermined number more than one of such sweeps are required to cause the potential of said bombarded portion to approach said equilibrium value, means effective after said predetermined number of sweeps for sweeping a high intensity reading electron beam across said surface to bring the potential of all of said bombarded portion to said equilibrium value during said last-named single sweep, and a grid positioned adjacent said surface for collecting secondary emission electrons produced by the high intensity reading electron beam, said grid having a signal output load coupled thereto.

3. An electron discharge tube circuit comprising an electron gun for projecting a beam of electrons, a potential storing target surface in the path of projection of said beam, means for applying a negative potential to said surface to initially bias the same, means repetitively sweeping said beam across said surface at a predetermined speed, means for producing an electron beam voltage with respect to the bias potential of said surface in the region necessary to give a secondary emission ratio greater than unity for said target so that the beam bombarded portion of said target tends to charge positively with respect to said bias potential toward a predetermined equilibrium potential value substantially different from said bias potential, means for controlling the writing intensity of said beam to a value substantially below that required to bring the potential of said bombarded portion to said equilibrium value in a single sweep thereacross, whereby a predetermined number more than one of such sweeps are required to cause the potential of said bombarded portion to approach said equilibrium value, and means effective after said predetermined number of sweeps for sweeping a high intensity reading electron beam across said surface to bring the potential of all of said bombarded portion to said equilibrium value during said last-named single sweep, said means for applying a negative potential to said surface comprising means for restoring said target surface to its initial bias value following reading of said surface.

4. An electron discharge tube circuit comprising an electron gun for projecting a beam of electrons, a potential storing target surface in the path of projection of said beam, means for applying a negative potential to said surface to initially bias the same, means repetitively sweeping said beam across said surface at a predetermined speed, means for producing an electron beam L9 voltage with respect to the bias potential of said surface in the region necessary to give a secondary emission ratio greater than unity for said target so that the beam bombarded portion of said target tends to charge positively with respect to said bias potential toward a predetermined equilibrium potential value substantially different from said bias potential, means for controlling the writing intensity of said beam to a value substantially below that required to bring the potential of said bombarded portion to said equilibrium value in a single sweep thereacross, whereby a predetermined number more than one of such sweeps are required to cause the potential of said bombarded portion to approach said equilibrium value, and means effective after said predetermined number of sweeps for sweeping a high intensity reading electron beam across said surface to bring the potential of all of said bombarded portion to said equilibrium value during said last-named single sweep, said means for applying a negative potential to said surface comprising a ood gun for restoring said target surface to its initial bias value following reading of said surface.

5. An electron discharge tube circuit comprising an electron gun for projecting a beam of electrons, a potential storing target surface in the path of projection of said beam, means for applying a negative potential to said surface to initially bias the same, means repetitively sweeping said beam across said surface at a predetermined speed, means for producing an electron beam voltage with respect to the bias potential of said surface in the region necessary to give a secondary emission ratio greater than unity for said target so that the beam bombarded portion of said target tends to charge positively with respect to said bias potential toward a predetermined equilibrium potential value substantially different from said bias potential, means for controlling the writing intensity of said beam to a value substantially below that required to bring the potential of said bombarded portion to said equilibrium value in a single sweep thereacross, whereby a predetermined number more than one of such sweeps are required to cause the potential of said bombarded portion to approach said equilibrium value, and means effective after said predetermined number of sweeps for sweeping a high intensity reading electron beam across said surface to bring the potential of all of said bombarded portion to said equilibrium value during said last-named single sweep, said means for applying a negative potential to said surface comprising a fiood gun biased below the secondary emission ratio of said surface for restoring said target surface to its initial bias value following reading of said surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,280,191 Hergenrother Apr. 2l, 1942 2,410,233 Percival Oct. 29, 1946 2,430,038 Wertz Nov. 4, 1947 2,451,005 Weimer et al. Oct. 12, 1948 2,459,319 Hansell Jan. 18, 1949 2,508,408 Liebson May 23, 1950 2,524,837 Russell et al. Oct. 10, 1950 2,563,500 Snyder, Jr. Aug. 7, 1951 

